The present invention relates to a direct drive transmission for heavy equipment and more particularly relates to a multi speed transmission with a two speed underway shift capability coupled with a plurality of speed gears engageable while the vehicle is stopped to provide varying power output and speed combinations. A reverse section is also provided which may be coupled through the plurality of speed gears.
Vehicle transmissions take many forms, each form generally dictated by the use of which the vehicle is put. In many vehicles, a wide speed range is important, consequently a multitude of possible underway shifts must be provided to take the vehicle from a stopped condition to the maximum and relatively high speed. In some cases this underway shift capability is in part avoided through the use of a torque converter. On the other hand, some vehicles require minimum underway shift capability as they operate at relatively constant and slow speeds or through a relatively small range of speeds. In these vehicles, the primary purpose of a transmission is to provide maximum power output over a relatively small range of output speeds.
In all cases, the goal is to operate the vehicle engine at its most efficient speed. Such a goal is, of course, unobtainable when the vehicle must be taken from a stopped condition to an underway condition, or similarly pass from a forward direction to a reverse direction. Accordingly, a compromise must be reached in the transmission to allow for varying speed conditions with a minimum deviation from optimum operating speed of the engine.
Use of a torque converter can provide some of this capability, however torque converters generally are inefficient at high input and low output speeds and thus are not adaptable to use in certain types of construction equipment. At the opposite end of the scale is a direct drive gear arrangement in which a minimum amount of friction loss occurs and a maximum amount of power transmission may take place. Unfortunately, a direct drive gear arrangement requires either a complex clutching assembly to engage the input and the output members while underway or the drive train must be engaged before the associated input and output shafts are rotated.
Planetary gear arrangements, which include a driving member, a reaction member and a driven member, have been found to be useful. Such an arrangement operates without severe friction losses while providing discrete reduction ratios similar to a direct drive gear arrangement. Furthermore, a planetary transmission allows the reaction member to be stopped or interconnected with another rotating member through the use of clutches or brakes without requiring the input and output members of the planetary gear arrangement to be stopped. Thus the underway clutching requirements for direct drive gearing previously mentioned are reduced. In addition to these advantages, it is well-known that in a typical planetary gear arrangement which includes a sun gear, or planet carrier having a plurality of planet gears mounted thereon and intermeshing with a sun gear and a ring gear intermeshing with a planet gear, when utilizing the planet carrier as the reaction member, the direction of rotation of the output member is reversed relative the input member. Thus planetary arrangements provide a compact reduction and reversing capability not found in other arrangements.
Combinations of direct drive gear arrangements and planetary arrangements provide a transmission with the best features of both arrangements. Such combination transmissions are particularly appropriate to the construction type vehicle to provide limited output speed range at or near maximum engine efficiency.